Digital Printing

3 02 2012

The idea of digital  printing on textiles has been around for some time.  Carpet inkjet printing machines have beenused since the early 1970s.  Digital ink jet printing of continuous rolls of textile fabrics was shown at ITMA in 1995.   Again at ITMA in  2003, several industrial inkjet printers were introduced to the marketplace which made digital printing on textiles the new industry standard.  These new generation machines had much higher outputs, higher resolution printing heads, and more sophisticated textile material handling systems allowing a wide varieof fabrics to be printed.

One reason for the comparatively slow growth of digital printing on textiles may be related to the  extreme demands of the textile applications.  Although ink-jet printing onto fabric works in fundamentally the same way as any office type ink-jet prints onto paper, fabric has always been inherently more difficult to print due to its flexible nature.  The level of flexibility varies from warp to weft and with each degree around the bias, so guiding the fabric under digital printer heads has proven to be very challenging.  Other challenges:

  • There are many  types of synthetic and natural fibers,   each with its own ink compatibility characteristics;
  • in addition to dealing with a fabric that is stretchable and flexible, it is often a highly porous and textured surface;
  • use requirements  include light fastness, water fastness (sweat, too) through finishing operations and often outdoor use, heavy wear, abrasion, and cleaning;
  • the fabric not only has to look good but to feel good too;
  • fabric has much greater absorbency, requiring many times the ink volume compared with  printing on papers.

Before any printing is carried out, the designs need to be developed in a digital format that can be read by the printers. Thus, all development has to be based on co-operation between the design software companies, the ink manufacturers and the printing machine developers.

In the face of such odds, digital textile printing is happening.  And how!  Digital inkjet printing has become one of the most important textile production printing technologies and is, in fact, transforming the industry. It has been influencing new workflows, business plans and creative processes. The opportunities for high-value digital printer applications are so large that many hardware and chemistry vendors are investing heavily in textile and textile-related products and systems. Between 2000 and 2005 digitally printed textile output rose by 300% to 70m square metres.[1]  This is still less than 1% of the global market for printed textiles, but  Gherzi Research, in a 2008 report, suggests the growth of digital printing on fabrics to be more than 20% per year.  This growth is largely driven by the display/signage sector of the market;[2] it is only recently that interior designers, seeking unique solutions for their clients, have been turning to digital printing.

Digital processes have become so advanced that it is becoming very hard to tell digitally printed fabric apart from fabric printed the traditional way – although for my money, they’ll never replicate the artisanal hand crafted quality of hand screened or hand blocked prints, where the human touch is so delightfully evident.  The lower energy, water and materials consumption means that more printers are switching to digital as it becomes competitive for shorter runs.   Although there are many advantages already to digital printing, the few downsides, such as lower production speeds compared to rotary screen printing and high ink costs are both changing rapidly.

As with traditional screen printing technologies, the variables in digital technologies are as varied as in screen printing,  with additional complexity of computer aided technologies requiring changes from the design stage onward.   Digital textile printing output is a reflection of the design and color management software (such as Raster Image Processing or  RIP) that provides the interface between the design software and the printer, the printer itself, the printing environment, the ink, the fabric, the pre-treatment, the post-treatment and last, but not least, the operator.

This print method is being heavily touted as the “greenest” option.  Let’s find out why they make these claims.

In theory, inkjet technology is simple – a printhead ejects a pattern of tiny drops of ink onto a substrate without actually touching it. Dots using different colored inks are combined together to create photo-quality images.  There are no screens, no cleanup of print paste, little or no wastage.

In practice, however, it’s a different story.  Successful implementation of the technology is very complex. The dots that are ejected are typically sub-micron size, which is much smaller than the diameter of a human hair (70 microns);  one square meter of print contains over 20 billion droplets! [3] They need to be positioned very precisely to achieve resolutions as fine as 1440 x 1440 dots per inch (dpi).  Since the inks used must be very fluid so as to not clog the printheads, nanotechnology is a huge part of the ink development.  In fact, according to Xennia, a world leader in digital printing inks, “microfluidic deposition systems are a key enabler for nanotechnology”.  This precision requires multi-disciplinary skills –  a combination of careful design, implementation and operation across physics, fluid mechanics, chemistry and engineering.

There are two general designs of ink jet printers:  continuous inkjet (CIJ)  and drop-on-demand (DOD). As the names imply, these designs differ in the frequency of generation of droplets.

In continuous ink jet printers, droplets are generated continually with an electric charge imparted to them. As shown schematically in Figure 1, the charged droplets are ejected from a nozzle. Depending upon the nature of the imposed electric field, the charged droplets are either directed to the media for printing, or they are diverted to a recirculation system. Thus, while the droplets are generated continuously, they are directed to the media only when/where a dot is desired. Historically, continuous ink jet printing has enjoyed an advantage over other inkjet technologies in its ability to use inks based on volatile solvents, allowing for rapid drying and aiding adhesion on many substrates. The disadvantages of the technology include relatively low print resolution, very high maintenance requirements and a perception that CIJ is a dirty and environmentally unfriendly technology due to the use of large volumes of volatile solvent-based fluids. Additionally, the requirement that the printed fluid be electrically chargeable limits the applicability of the technique.

FIGURE 1.Continuous ink jet (schematic). Charged droplets leaving the nozzle are directed either toward a substrate or toward an ink recirculation system, depending upon the imposed electric field.

In DOD ink jet printers, droplets are generated only when they are needed. There are two subcategories in DOD jet printers:

  • The droplets can be generated by heating the ink to boil off a droplet,  called thermal ink jet.  Thermal inkjet technology (TIJ) is most used in consumer desktop printers but is also making some inroads into industrial inkjet applications. In this technology, drops are formed by rapidly heating a resistive element in a small chamber containing the ink. The temperature of the resistive element rises to 350-400ºC, causing a thin film of ink above the heater to vaporise into a rapidly expanding bubble, causing a pressure pulse that forces a drop of ink through the nozzle. Ejection of the drop leaves a void in the chamber, which is then filled by replacement fluid in preparation for creation of the next drop.  The advantages of thermal inkjet technology include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead costs and the potential for high native print resolution. The disadvantages of the technology are primarily related to limitations of the fluids which can be used. Not only does the fluid have to contain a material that can be vaporised (usually meaning an aqueous or part-aqueous solution) but must withstand the effects of ultra high temperatures. With a poorly designed fluid, these high temperatures can cause a hard coating to form on the resistive element (kogation) which then reduces its efficiency and ultimately the life of the printhead. Also, the high temperature can damage the functionality of the fluid due to the high temperatures reached (as is the case with certain biological fluids and polymers).
    • Alternatively, the droplets can be ejected mechanically through the application of an  electric stimulation of a piezoelectric crystal (usually lead zirconium titanate)  to elicit a deformation.  This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle.   This method is shown in Figure 2. Piezo  drop-on-demand inkjet technology is currently used for most existing and emerging industrial inkjet applications. In this technology, a piezoelectric crystal (usually lead zirconium titanate) undergoes distortion when an electric field is applied. This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle. There are many variations of piezo inkjet architectures including tube, edge, face, moving wall and piston, which use different configurations of the piezo crystal and the nozzle. The advantages of piezo inkjet technology include the ability to jet a very wide variety of fluids in a highly controllable manner and the good reliability and long life of the printheads. The main disadvantage is the relatively high cost for the printheads, which limits the applicability of this technology in low cost applications.
    • FIGURE 2.Piezoelectric drop on demand ink jet (schematic). In a DOD ink jet printer, upon application of a mechanical pulse, the ink chamber is deformed. This results in the ejection of a droplet toward the substrate.

As with screen printing, there are steps other than printing which are often overlooked:   the first step in digital printing is the pretreatment of the fabric.  Because many chemicals and/or auxiliaries cannot be incorporated into the printing ink, they must be applied to the fabric during the pretreatment. The entire process has to be designed to control bleeding, but also to achieve the hand, color, and fastness required in  the finished textile. For basic fabric pretreatment, the elements of this solution can include:

  • Antimigrants – To prevent migration of ink and prevent “bleeding.”
  • Acids/Alkalis – To support reactions of acid and reactive inks, respectively.
  • Urea/Glycols – To increase moisture content of the fabric, giving high, even fixation of the inks.
  • “Effects” Chemicals – Vary widely in purpose. Although there are too many effects to mention here, they can include chemicals to improve the brightness of the prints, water and stain repellants, UV absorbers to improve the fabric’s resistance to sunlight, fabric softeners/stiffeners, even antimicrobials to provide resistance to mildew and germs.

Many patented and proprietary formulations for pre treatment exist, ranging from simple formulations of soda ash, alginate and urea to more sophisticated combinations of cationic agents, softeners, polymers and inorganic particulates such as fumed silica. Many of these have been aimed at fashion fabrics such as cotton, silk, nylon and wool. The processing of the fabric during pretreatment is also an important factor in producing a superior finished printed fabric. Fabrics must be crease-free and even in width. Some producers provide fabrics that are backed with removable paper to allow companies with graphic printers that have been retrofitted with textile inks to print fabrics. This paper, and the adhesive that holds it to the fabric, must be properly applied so that the paper can be removed easily from the fabric.

Inks used in digital printing are thinner than those used for traditional printing, so the fabric also needs to be prepared by soaking it in a thickening agent.  This agent reacts to moisture by swelling.  As soon as a drop of dye touches the pre treated fabric, the thickener will swell up, keeping the dye in its place.  Without this agent, the dye would run and bleed on the fabric.

Inkjet inks must be formulated with precise viscosities, consistent surface tension, specific electrical conductivity and temperature response characteristics, and long shelf life without settling or mould-growth. The inks, made up  of pigments or dyestuffs of high purity,  must be milled to very fine particle size and distributed evenly in solution.  In addition, further properties such as adequate wash-, light- and rub-fastness are necessary.

Inkjet inks contain dyes or pigments but like screen printing inks they contain other things too:

  • Surfactants
  • Liquid carriers (water or other solvents)
  • Binders
  • Rheology modifiers
  • Functional materials
  • Adhesion promoters
  • Other additives
  • Colorants (dyes or pigments)[4]

The inks used in digital printing today have comparable color performance and fastness as compared to traditional screen printing inks.  They fall into four general categories:

  1. Water based – can contain glycol plus pigments or dyes.  These inks are designed to run specifically in printers with thermal and piezo-electric print-heads.  Dyes used include:

                  Reactive dyes, particularly suited to cotton, viscose and other cellulosic materia

                 Acid dyes, used for wool, silk and nylon.

                 Disperse dyes are used for synthetics like polyester and nylon.

  1.   Pigments (as well as disperse dyes)  present a more difficult set of problems for ink makers. Both exist in    water as a dispersion of small particles. These inks must be prepared with a high degree of expertise so that the particles will not settle or agglomerate (flocculate) and clog the printheads. The particle size must have an average of 0.5 micrometer and the particle size distribution must be very narrow with more than 99% of the particles smaller than 1 micrometer in order to avoid clogging of the nozzles. The major outstanding problem with the use of pigments in inkjet systems is how best to formulate and apply the resins which are required to bond the pigment particles to the fabric surface. Several different approaches, from coating pigment particles with advanced surfactants, to spraying resin through a separate jet head to screen printing binder over an inkjet  printed color have been suggested.
  2. Solvent based – Solvent-based inks are relatively inexpensive and have the advantage of being able to produce good vivid colors. However, their main ingredients are volatile organic compounds (VOCs) which produce harmful emissions. These inks need to be employed in machines which have ducting to extract the solvents to atmosphere. It is possible to remove the VOC’s using activated carbon filters without ducting to outside the building however you still have to dispose of the solvent laden graphite. Fabrics produced using solvent-based inks have a strong odor. The higher the level of the solvent, the greater the keying, or bonding, with the material’s surface to give a durable finish. Types of solvent range from eco-solvent, low and mild solvent through to hard or full solvent. The term eco-solvent does not necessarily mean less environmentally damaging than conventional solvent, as discussed in the post entitled “Textile Printing and the Environment”.
  3. Oil based – requires the use of a printer which is compatible; otherwise similar to water and solvent based inks.  Oil-based inks are less commonly used, but offer very reliable jetting since the ink does not evaporate.
  4. UV curable – generally made of synthetic resins which have colored pigments mixed in.  Curing is a chemical reaction that includes polymerization and absorption by the fabric. UV inks consist of oligomers, pigments, various additives and photoinitiators (which transfer the liquid oligomers and monomers into solid polymers).
  5. Phase change –  ink begins as a solid and is heated to convert it to a liquid state. While it is in a liquid state, the ink drops are propelled onto the substrate from the impulses of a piezoelectric crystal. Once the ink droplets reach the substrate, another phase change occurs as the ink is cooled and returns to a solid form instantly.

Once you have digitally printed the fabric, you must perform some process to fix the ink. What process this is depends on the type of ink you used.  Each dye type needs a specific finishing agent.

Finally, the fabric needs to be washed to remove the excess dye and thickening agents.  Fabrics are washed in a number of wash cycles at different temperatures to make the print washfast.

So at the end of this process, you can see that there is no real difference in the amount – or kinds –  of chemicals used, except perhaps those lost through wastage.  So what exactly are the green claims based on?

The traditional printing industry produces large amounts of waste – both dyes/pastes and water, and it has high energy useage.  There are also large space reqirements to operate presses, which produce a lot of noise.  In a project sponsored by the European Union’s LIFE Program, an Italian printing company,  Stamperia di Lipomo, transferred from conventional printing to digital.[5]  They found that these new digital presses lowered water, energy and materials consumption significantly.  The following reductions were achieved:

  • Production space required by 60%
  • Noise by 60%
  • Thermal energy usage by 80%
  • Wastewater by 60%
  • Electricity consumption by 30%
  • By-production of waste dyes = eliminated entirely

Digital printing has other advantages, which include:

  • Minimal set up costs – short runs and samples are economical – so traditional mill minimums can be avoided.  Costs per print are the same for 1 or 1000000.
  • There is no down time for set up – the printer is always printing – so there is also increased productivity.
  • Faster turnaround time – and very fast design changes.  Turnaround time for samples can be reduced from 6 to 8 weeks to a few days.
  • Print on demand, dramatically reducing time to market.
  • Just-in-time customization or personalization
  • Theoretically no limit on number of colors.
  • Decreases industrial waste and print loss.

The disadvantages most often cited, that of high cost of inks and shorter printing speeds, are quickly being overcome by the manufacturers.

One concern I have is that of the use of nanotechnology, which seems to be an inextricable part of the equation.  Already nanotechnology is enabling manufacturers to offer functional finishes in post processing, such as stain and water repellants, fire retardants, and UV blocking .  It is also being used in smart clothing:  To harness the energy of the sun, flexible thin film modules are sewn onto clothes. However, since they show clearly when sewn,  digital textile printing makes these modules inconspicuous.[6]

The traditional industry still looks at digital textile printing parameters from the context of what it “can’t do,” compared to conventional printing (much of which is already history).  For a much smaller group of designers, textile artists, fine artists, costumers, wide-format printers and the like, this technology is much more about what it “can do” to provide to provide products and services the market has never before seen. For these people, textile printing offers parameters not available with conventional printing:  unlimited repeat size, tonal graphics, engineered designs that cross several seam lines, quicker samples, customization and short-run production.  And the use of the technology is beginning to catch the imagination of more and more textile designers, as they realize that their old reaction to computer generated graphics (dismissive to say the least)  is truly outdated.  Claire Lui, Print magazine associate editor, points out that in  ultra-custom milieus, design and printing become more like art than common manufacturing.

The traditional textile industry needs to understand that, in the same way the Internet is not going to replace the television as a form of entertainment or information, this new digital technology isn’t about replacing existing processes , but rather about leveraging the expanded parameters to offer new niche products and services.  And we must remember too that digital printing is not the panacea it’s touted to be for the environment, though it seems to have less of a pollution footprint than traditional screen or rotary printing.


[3] Xennia

[4] Yeong, Kay, “Inkjet Printing: Microfluidics for the Nanoscale”; http://www.xennia.com/Xennia/uploads/ppp-InkjetPrintingMicrofluidicsfortheNanoscale-Jun2010.pdf





Textile printing and the environment

27 01 2012

Given the large size of the printing industry, and the extraordinary volume of chemicals it consumes, it is not surprising that it also generates a significant amount of pollution.  Gaseous emissions have been identified as the second greatest pollution problem (after effluent quality) for the textile industry – and these are largely generated in printing. Speculation concerning the amounts and types of air pollutants emitted from textile operations has been widespread but, generally, air emission data for textile manufacturing operations are not readily available. Air pollution is the most difficult type of pollution to sample, test, and quantify in an audit.[1]  According to the U.S. EPA, the printing industry releases 99% of its total Toxic Release Inventory (TRI) poundage to the air, while the remaining one percent of releases are split between water and land disposal. This release profile differs significantly from other TRI industries which average approximately 60% to air, 30% to land, and 10% to water release respectively. Average VOC emissions per textile print line are 130 Mg (tons)/year for roller and 29 Mg/year for flat and rotary screen.[2]

In 1995, more than 41 million pounds of toxic compounds were transferred or released into the environment by the printing industry in the United States alone.  The table below shows some of the  polluting chemicals used by the textile printing industry.  All ten are petroleum-derived.

Chemical Releases and   transfers in millions of pounds
Toluene 4.2
Methyl Ethyl   Ketone 6.3
Glycol Ethers 0.4
Xylene 0.2
Methyl   Isobutyl Ketone 0.6
Methanol 0.3
1,1,1-Trichloroethane 0.3
Ethylene   Glycol 0.5
Dichloromethane 0.1

Source: EPS: Profile of the Textile Industry, EPA/310-R-97-009, September 1997

These VOC emissions are high because of the great quantity of solvents used in the industry. The volatility that helps minimize ink drying times also presents a health and safety risk.  The solvents used in the printing pastes are typically respiratory, skin and eye irritants. But there are also more dire consequences – for example, a study done on Indian printing working has found abnormal changes in their chromosomes.(3)  With such a high percentage of the paste being volatile, solvent vapors will be released during printing and will be present throughout the printing production area. Also, the fabric will continue to off-gas solvents after the material has been printed, especially if it has been rolled up.  The Sector Notebook  gives a short synopsis of these chemicals, and I’ve excerpted a few here:

  • Toluene, although used primarily as a solvent,  is also used throughout printing for cleanup purposes. Toluene contributes to the formation of ozone in the atmosphere; studies have shown that unborn animals were harmed when high levels of toluene were inhaled by their mothers, although the same effects were not seen when the mothers were fed large quantities of toluene. Note that these results may reflect similar difficulties in humans.
  • Data on ethylene glycol mono-n-butyl ether is used to represent all glycol ethers because it is the most commonly used glycol ether in printing.  It can leach into ground water, and reacts with photochemically produced hydroxyl radicals.  For humans, moderate exposure may cause central nervous system depression, including headaches, drowsiness, weakness, slurred speech, stuttering, staggering, tremors, blurred vision, and personality changes. These symptoms are such that a patient, in the absence of an accurate occupational history, may be treated for schizophrenia or narcolepsy.
  • Methyl ethyl ketone contributes to the formation of air pollutants in the lower atmosphere; breathing “moderate amounts” for short periods of time can cause adverse effects on the nervous system ranging from headaches, dizziness, nausea, and numbness in the fingers and toes to unconsciousness; repeated exposure to moderate to high amounts may cause liver and kidney effects.

Everybody is now talking about “water based” inks, as if that’s the answer to help reduce these emissions.  So, let’s investigate these inks and see what “water based” means, and what the concerns may be.

There are three general types of texile inks (or pastes, as we referred to them in Printing – Part 2):

  • traditional solvent-based inks
  • water-based inks
  •  plastisol inks

The two inks used most often in textile printing are water-based (used mostly for yardgoods) and plastisol  inks (used for printing finished goods, such as T shirts, sweatshirts, tote bags).

SOLVENT-BASED INKS:   The solvent has two primary functions: 1) to carry the ink to the substrate, and 2) to evaporate quickly, leaving only the ink film on the substrate. While water is a solvent, the name solvent-based ink is used to describe a highly volatile solvent such as 2-butoxyethyl acetate, cyclohexanone and n-butyl acetate.

Solvent based inks are considered the least environmentally friendly due to the  highly volatile solvents given off during printing and drying. The petroleum-based binder used in many solvent-based inks could be replaced with renewable resources such as vegetable oil or soy. The downsides are that the inks dry very slowly are less durable, and still contain solvents emitting VOCs during printing.

Therre are now inks on the market called Eco Solvent inks.  To most people, “eco” means ecological, and to be fair these inks are not as nasty as full solvent inks.   But these inks generally contain glycol esters or glycol ether esters – both derived from mineral oil – hardly a renewable resource or an ecologically sound process.  Tony Martin, president of Lyson Inc. suggested we call these inks “mild” vs. the “aggressive” traditional solvent inks.  Also since these inks are generally used to print onto PVC, the green claim sorta gets overlooked by the elephant in the substrate.

WATER-BASED INKS:  These use water as the main solvent.  But that does not mean that water is the ONLY solvent used.  It is significant to note that many water base inks contain “co-solvents” which may even be petroleum based solvents.[4] ( See Printing – Part 2 for components of typical water and solvent based inks.) The reason these co-solvents are used varies, but a main reason is to decrease the time and heat necessary to cure the ink on the fabric.

There are two types of water-based inks: Traditional (air dry) ink and Discharge ink.

  • Traditional air dry ink soaks into the cloth and binds with the fibers providing good colorfastness and wash ability.
  • Discharge ink removes the original dye/color from the garment and replaces it with a color/pigment. Discharge inks are now available in formaldehyde free formulations, such as the Oasis Series by Wilflex, making them safer for the user and the environment.

Water based inks are usually less expensive than solvent-based inks and are similar in quality, gloss, and adhesion.

Many printers observe that water-based inks have more vibrant colors and print more crisply than their solvent-based counterparts. The sharper definition possible with water-based inks allows printers to use finer dot patterns in screened process printing. Water-based inks are a good choice when a “soft hand” is desirable. (A soft hand is the condition where the ink film cannot easily be felt with the hand when passed across the surface of the fabric. This affect is often used as an argument for why water-based is preferable to plastisol  because plastisol has more of a hand than water-based, and this is considered a consumer turn off.)

These inks are inexpensive and easy to manufacture. In fact, with some experience and the proper equipment, printers can even make them in small batches from basic natural components. They have a very limited shelf life and are difficult to re-use, so they generate more wasted ink than regular plastisols or more complex, manufactured water-based inks. While this type of water-based ink is considered a very green alternative, this extra waste is something to consider.

An advantage often cited for water-based inks is that they do not require organic solvents when cleaning the presses.  But there is a common misconception that because water can be used for cleaning screens, squeegees and tools, that the waste water can just be discharged into the sewer. However, the water-based ink is not just water. There are pigments, binders, thickeners, and sometimes, even co-solvents in the ink residue.

Many printers believe that screen printing using water based inks is the cutting edge of textile printing.  So why isn’t everybody using them?

Water-based inks cure as water evaporates out of the ink so they have a longer – and more difficult –  drying time than plastisol inks. This means that the water — along with whatever in the ink evaporates with the water — enters the environment.

If using water-based ink, the facility must have the drying capacity to remove the water. The dryers used for water-based printing tend to be larger than those needed for plastisol.  In plastisol printing, the ink film must only reach the cure temperature for a brief moment. With water-based ink, the temperature must be reached and then held until all of the solvent (water) is removed. There are water-based inks that will air dry but they are usually only acceptable for craft level printing as the room required for curing greatly reduces productivity.
Finally, all water-based inks can start to dry out during use, so care must be taken to prevent the ink from drying on the screen.  If water based ink is left in open mesh for even a short period of time, it can clog the mesh and ruin the screen. Practiced waterbased ink printers must always be conscious of how long a screen sits between prints to prevent the ink from “drying in”. While modern water-based inks are less prone to this phenomenon, it is still a concern.  In addition, overall shelf life is limited.

There have been major improvements in manufactured water-based inks in recent years. These newer inks have a number of performance advantages over the basic water-based inks discussed above and are as potentially eco-friendly and sustainable as any alternative. For example, they resist drying, and remain useable far longer than traditional water-based and discharge inks. They can be re-constituted with water — and additional binder, if needed — which can cut back on waste. Shelf life of these newer water-based inks is substantially longer as well because the manufacturers have developed technology to encapsulate the water in the ink in such a way that it does not readily evaporate until printed.

Much like traditional plastisol, these water-based inks are sold ready to use as colors or underbases and have a thicker viscosity that yields greater opacity on finished prints. They can be reduced with water and other modifiers for a softer hand.

PLASTISOL INKS:  Plastisol inks, commonly used for textile printing and especially for t-shirts, are a PVC-based ink composed of a clear, thick plasticizer fluid and PVC resin. The full name for PVC is polyvinyl chloride. The PVC life cycle results in the release of toxic, chlorine-based chemicals which end up as by-products such as carcinogenic and highly toxic dioxin and PCB.  The major health concern about plastisol inks is not that they are PVC-based but that they contain phthalates. Phthalates are added to PVC plastics to transform a hard plastic into a soft, rubbery plastic by allowing the long polyvinyl molecules to slide against each other instead of rigidly binding together. These phthalates used in plastisol ink to make the PVC flexible are also carcinogenic and much research has been done which substantiates the damage phthalates do to us,  especially to fetuses and newborns.[5] They are released into the environment during the printing and curing of the ink and they will continue to exhaust toxins when exposed to a radiant heat source, such as a dryer or even sunlight.  Plastisol inks contains virually no solvents at all.

Plastisol does not “dry”. In order for a compound to dry, there must be evaporation of some kind of solvent.  These inks typically contain less than 1% VOC.  Some water based plastisol inks can contain about 30% VOCs.[6] Since plastisol has little or no solvent, it cannot dry. Plastisol is a thermoplastic ink  – meaning it is necessary to heat the printed ink film to a temperature high enough to cause the molecules of PVC resin and plasticizer to cross-link (i.e., bond to the fabric)  and solidify, or cure.  Cross-linking agents must be used to effect the bonding, and  formaldehyde is often a necessary component of these cross linkers.  The temperature at which most plastisol for textile printing cures at is in the range of 300 °F to 330 °F.  Because of this characteristic, plastisol can be left in screens for long periods of time without clogging the mesh, the lids can be left off of the ink containers (although keeping them covered is a good practice to keep lint and dirt out of the ink). And ink left at the end of the job can be returned to the container for reuse without any adverse affects. This last practice is a great benefit in reducing waste product.  It is ready to use right out of the container more than 90% of the time. In most applications, it can be printed wet-on-wet, which allows for increased production speeds. It comes in formulations that can be printed on light and dark fabrics.

Since Plastisol is a thermoplastic, it will remelt if it comes in contact with anything hot enough. For that reason, plastisol prints cannot be ironed. If an iron touches a print, it will smear the ink.

Plastisol ink also creates an ink film that can be felt with the hand. The higher the opacity of the ink, the greater the hand. This heavy hand is considered a disadvantage at the consumer level.

Because both PVC and phthalates are chemicals of concern, many companies are offering phthalate free plastisol inks. These non-phthalate inks are not as easy to work with as standard plastisols, but it is possible to use them to accomplish most of the common printing techniques. In addition to non-phthalate plastisols, there are some new acrylic-based screen printing inks that are sometimes referred to as non-PVC and non-phthalate plastisols. Why? Well, an acrylic-type resin replaces the PVC resins used in regular plastisol. Also, the plasticizer in acrylic inks is normally non–phthalate, making these inks an even more eco-friendly alternative.

With some experience, acrylic inks can be successfully made into high-density designs. The finished prints lack the soft finish of a standard high-density plastisol print, but this may be an acceptable compromise to some customers.

Acrylic inks are usually a little more costly than standard plastisols and are substantially more expensive than standard water-based inks.

The hazards of plastisol printing inks are not just to personal health but also to environmental health. Garments coated with plastisol inks do not decompose and they are difficult to recycle. The result is that you may soon grow tired of your Rolling Stones concert tee shirt and trash it, but it will live on in immortality in the local landfill. If clothing designed with PVC plastisol ink is incinerated, the trapped dioxins plus hydrochloric acid (a primary component of acid rain) are released into the atmosphere.

New inks have also been developed for digital printing, such as latex, resin and UV curable inks.  We’ll discuss them next week with digital printing.

Dr. Nicholas Hellmuth, of FLAAR (http://www.wide-format-printers.org/), writing in his January, 2011 blog, said of the proliferation of green claims by ink manufacturers: ” I would bet that 90% of these claims were misleading at best. I would bet that more than 50% of these claims are fraudulent and inaccurate… I looked at the MSDS of inks called water-based and almost gagged when I saw the chemical recipe, with the hazardous warnings.  If you make a list of the nasty chemicals that are   really in the ink, depending on what chemicals you consider unhealthy,   resin ink could potentially be considered less unhealthy than even   traditional water-based ink. In other words, there is a potential that   resin inks could be considered better than water-based inks. But there   are so many diverging opinions that I will be discussing this with other   ink chemists as I meet them during the expos early in this year (2011). ”

So you’d think that the major source of the emissions comes from using these inks – the printing process itself.  You’d be wrong:  the majority of emissions to the atmosphere from textile printing is from  the drying process, which drives off volatile compounds.  The largest VOC emission source is the drying and curing oven stack, which vents evaporated solvents to the atmosphere.  Another source of fugitive VOC emissions comes from the “back grey” (fabric backing material that absorbs excess print paste), which  is dried before being washed. In processes where the back grey is washed before drying, most of the fugitive VOC emissions from the back grey will be discharged into the waste water. In some roller printing processes, steam cans for drying printed fabric are enclosed, and drying process emissions are vented directly to the atmosphere.

As of the publication date of the EPA Sector notebook on the Printing and Publishing Industry (1995),  there was no add-on emission control technology for organic solvents used in the textile printing.

Another environmental hazards  in printing textiles comes in the screen and equipment cleaning steps – which use lots of water.  When you finish a printing run, for example, there are still approximately 1.5 gallons of printing paste in the system, predominantly in the tubes that run between the paste reservoirs and the screens. This  is simply rinsed out and flushed down the drain. If using plastisol inks, in order to emulsify the ink for easy removal from screens, squeegees, flood bars, spatulas, and work surfaces, it is necessary to use some type of solvent.   Solvents used to clean printing equipment include toluene, xylene, methanol, and methyl ethyl ketone (MEK). In addition, blankets used to transfer the ink-filled image to sheets of paper are cleaned with washes that contain glycol ethers and 1,1,1-trichloroethane (TCA). The type of solvent used depends largely on the equipment to be cleaned. For example, a blanket wash must dissolve ink quickly and dry rapidly with minimal wiping. Conversely, a solvent that is intended to clean a chain of ink rollers must evaporate slowly, to insure that it does not flash off before it has worked its way through all the rollers. Water based inks contain co-solvents, additives, dyes and/or pigments, which make the water clean up full of possibly hazardous materials.   All of these components must be washed thoroughly.

Irrespective of the type of inks used, all printers attempt to reclaim screens, which are a major cost item. Failure to reclaim screens and ruined screens cost on average $5,000-$10,000 per year. One study showed chemical reclamation cost between  $2 and $10  per average screen, while screen disposal cost just shy of $50. Screen reclamation is a particular challenge to screen printers, because inks and solvents cannot go down the drain and some of the chemicals used to reclaim mesh are restricted.   The waste water will contain particulates comprised of ink pigment, emulsion and emulsion remover.  Reclaiming screens involves these steps:

  1. Remove the paste:  Any and all excess paste in the screen should be “carded off” for reused on another job. The screen must then be washed to remove any remaining paste because the paste will interfere with the process of removing the stencil. Screen cleaning solvents are a source of VOC emissions.
  2.  Emulsion removal:  The stencil or emulsion is removed by spraying the screen with a solution of water and emulsion remover chemicals which is comprised mainly of sodium metaperiodate,  then rinsing the solution away with fresh water.
  3.  Haze or ghost image removal:  Finally, if any haze or “ghost image” remains, a haze remover must be applied. Some haze remover products are caustic and can damage or weaken the screen. Haze removers make screens brittle and tear easily, therefore only small amounts should be used. Ghost image is a shadow of the original image that remains on the screen caused by paste or stencil caught in the threads of the screen.

The best way to reduce VOCs during screen reclamaition are related to technology and best practices, such as using high pressure wash systems and modifying how chemicals are applied to the screens.

The waste ink and the solvent must be disposed of properly in order to minimize environmental impact.  There are three major areas of concern for this wastewater:

  • Heavy metals, which can be found in the residue of ink, can enter the sewer system and contaminate sewage sludge
  • Heavy concentrations of certain chemicals can disrupt the pH balance at the treatment plant and disrupt the bacterial systems essential to the sewage treatment process
  • Combinations of mixtures with low flash points can cause flammability concerns in the sewage system

Leftover print pastes cannot be allowed to enter the wastewater treatment system. It must be disposed of as a solid waste. Sites where sludge piles are used can have environmental problems with ground and groundwater contamination. These sludge storage areas should be equipped with waterproof linings to prevent this from occurring.

In fact, textile printing is becoming an important wastewater source as the water-based materials replace the organic solvents. The wastewaters originating from this operation are often strong and may contain toxics, although their volume is still quite low.[7]

The screen printing industry has been very proactive in the creation of products that can minimize the impact of these cleaning processes. Solvents are available that are “more” environmentally sensitive than the traditional petroleum based solventsCompanies are beginning to market biochemical cleaning solutions, inks and additives to replace current solvents or toxic chemicals– examples include the use of terpene d-limonene (derived from citrus fruit), coconut oil , soybeans, seaweed  and fatty amides. (8)  In addition, there are many types of filtration and cleaning systems available to capture inks and solvent residues to minimize the solids that are discharged into the sewer system.

Aside from improvements to the building itself and efforts to minimize water use and to use inks and paste effectively, there are some things every printer can do to reduce their environmental impact:

  • Minimize downtime on the press
  • Make rejects history
  • Maintain dryers – is it really worth saving money by buying that second hand dryer?  A new one is 30% more efficient, twice the price but the energy savings will pay the difference in 9 months.  An average printing line has a nominal power rating of 75 kW, most of which is required for the drying process.

[2] http://www.epa.gov/ttnchie1/ap42/ch04/final/c4s11.pdf

(3)  Sellappa, Sudha, et al; Genotoxic  Effects in Textile Printing Dye Exposed Workers by Micronucleus Assay, Asian Pacific Journal of Cancer Prevention, Vol 11, 2010;  pgs. 919-922,  http://www.apocp.org/cancer_download/Volume11_No4/c%20919-22%20Sellappa.pdf

[7] Kabdasli, M Gurel & Tunay, O., “Characterization and Treatment of Textile Printing Wastewaters”, Environmental Technology, Vol 21, Issue 10, 2000, pp. 114 – 1155

(8) http://www.pneac.org/sheets/all/biochemicals_for_the_printing_industry.pdf





Printing – part 3

19 01 2012

Yes, we’re still talking about the printing process!  As I warned you, it’s complicated.

For the past two weeks we’ve concentrated on the first two steps of the basic 5 steps in printing a fabric, which  are:

1. Preparation of the print paste.

2. Printing the fabric.

3. Drying the printed fabric.

4. Fixation of the printed dye or pigment.

5. Afterwashing.

So let’s look at the rest of the steps – drying, fixation and afterwashing.

Actually, the printing process begins even before passing  the fabric thru the printing presses, because the fabric must be conditioned.  The cloth must always to be brushed, to free it from loose nap, flocks and dust that it picks up while stored. Frequently, too, it has to be sheared by being passed over rapidly revolving knives arranged spirally round an axle, which rapidly and effectually cuts off all filaments and knots, leaving the cloth perfectly smooth and clean and in a condition fit to receive impressions of the most delicate engraving. Some figured fabrics, especially those woven in checks, stripes and crossovers, require very careful stretching and straightening on a special machine, known as a stenter, before they can be printed with certain formal styles of pattern which are intended in one way or another to correspond with the cloth pattern. Finally, all descriptions of cloth are wound round hollow wooden or iron centers into rolls of convenient size for mounting on the printing machines.

Immediately after printing, the fabric must be dried  in order to retain a sharp printed mark and to facilitate handling between printing and subsequent processing operations.

Two types of dryers are used for printed fabric, steam coil or natural gas fired dryers, through which the fabric is conveyed on belts, racks, etc., and steam cans, with which the fabric makes direct contact. Most screen printed fabrics and practically all printed knit fabrics and terry towels are dried with the first type of dryer, not to stress the fabric. Roller printed fabrics and apparel fabrics requiring soft handling are dried on steam cans, which have lower installation and operating costs and which dry the fabric more quickly than other dryers.

After printing and drying, the fabric is often cooled by blowing air over it or by passing it over a cooling cylinder to improve its storage properties prior to steaming, which is the process which fixes the color into the fabric.  Steaming may be likened to a dyeing operation.  Before steaming, the bulk of the dyestuff is held in a dried film of thickening agent.  During the steaming operation, the printed areas absorb moisture and form a very concentrated dyebath, from which dyeing of the fiber takes place.  The thickening agent prevents the dyestuff from spreading outside the area originally printed, because the printed areas act as a concentrated dyebath that exists more in the form of a gel than a solution and restricts any tendency to bleed.  Excessive moisture can cause bleeding, and insufficient moisture can prevent proper dyestuff fixation.  Steaming is generally done with atmospheric steam, although certain tyepes of dyestuffs, such as disperse dyes, can be fixed by using superheated steam or even dry heat.  In a few instances, acetic or formic acid is added to the steam to provide the acid atmosphere necessary to fix certain classes of dyes.  Temperatures in the steamer must be carefully controlled to prevent damage from overheating due to the heat swelling of the fabric, condensation of certain chemicals, or the decomposition of reducing agents.

Flash aging is a special fixation technique used for vat dyes. The dyes are printed in the insoluble oxidized state by using a thickener which is very insoluble in alkali. The dried print is run through a bath containing alkali and reducing agent, and then directly into a steamer, where reduction and color transfer take place.

After steaming, the printed fabric must not be stored for too long prior to washing because reducing agent residues may continue to decompose, leading to heat build up in the stacked material and defective dyeing or even browning of the fibers. If a delay of several hours is anticipated before the wet aftertreatment the fabric should be cooled with air (called “skying”) to oxidize at least some of the excess reducing agent.

Finally, printed goods must be washed thoroughly to remove thickening agent, chemicals, and unfixed dyestuff.  Washing of the printed material begins with a thorough rinsing in cold water.  After this, reoxidation is carried out with hydrogen peroxide in the presence of a small amount of acetic acid at 122 – 140 degrees F. A soap treatment with sodium carbonate at the boiling point should be begun only after this process is complete. This washing must be carefully done to prevent staining of the uncolored portions of the fabric.  Drying of the washed goods is the final operation of printing. 

And there you have it – a beautifully printed fabric that you can proudly display. Bet you know the subject of the next post – the environmental consequences of all this. Stay tuned.





Printing – part 2

13 01 2012

Bear with me – I’ll eventually get to the environmental aspects of printing – including digital printing.  But I think it’s important to know the basic steps and processes in order to be able to understand green claims.  So there will still be a Printing – part 3 before we get to the environmental topics.

Specific fiber materials and dye types interact with each other in well defined ways, and it is these interactions that determines the best composition of a printing paste or ink.  The preparation of this paste is one of the most important steps in printing. (note:  paste and ink seem to be interchangeable names for the same substances).

It requires a set of special characteristics  – one of the most important is that the paste be viscous (like paint or pudding).

Printing paste ready to use.

This quality is called “flow”.  The choice of an agent to create this flow (called a thickening agent) is a critical component. In addition, each printing method we talked about last week (flat bed, screen or rotary), as well as the nature and sequence of fixation and aftertreatment steps  requires a specific kind of printing ink or paste.

For direct printing, a printing paste is prepared by dissolving the dyes in hot water to which is added urea and a solvent (ethylene glycol, thioethylene glycol, sometimes glycerine or a similar substance – and sometimes water).   This solution is stirred into a thickener that is easily removed by washing.  Small amounts of oxidizing agents are added.[1]

After making the printing paste, it is essential to strain or sieve all colours in order to free them from lumps, fine sand, and other foreign objects, which would inevitably damage the highly polished surface of the engraved rollers and result in bad printing. Every scratch on the surface of a roller prints a fine line in the cloth, and too much care, therefore, cannot be taken to remove, as far as possible, all grit and other hard particles from every color.

The straining is usually done by squeezing the paste through filter cloths as artisanal fine cotton, silk or industrial woven nylon. Fine sieves can also be employed for pastes that are used hot or are very strongly alkaline or acid.

All the necessary ingredients for the paste are metered (dosed) and mixed together in a mixing station. Since between 5 and 10 different printing pastes are usually necessary to print a single pattern (in some cases up to 20 different pastes are applied), in order to reduce losses, due to incorrect measurement, the preparation of the pastes is done in automatic stations. In modern plants, with the help of special devices, the exact amount of printing paste required is determined and prepared in continuous mode for each printing position, thus reducing leftovers at the end of the run.

There are two main types of paste used:

  1. Pigmented emulsions: Pigmented emulsions are suitable for all fiber types,  they are able to dry by evaporation at room temperature and are able to be cured at 320 degrees F for 2 – 3 minutes, which achieves washing and drycleaning fastness.  A typical formulation of a pigment emulsion printing paste is:

COMPONENTS

RATIO

Water

10%

Emulsifier

1%

Thickener

4%

White spirit

62%

Catalyst solution

3%

Binder

15%

Pigment dispersion

5%

Pastes which are entirely water-based are obtained by replacing the white spirit  with  water.

  1. Plastisol printing pastes :  based on a vinyl resin dispersed in plasticizer; characterized by virtually 100% non-volatility (no solvent is present); used frequently for printing on dark or dark-colored fabrics.  Components of plastisol printing pastes consist of
    1. PVC homopolymer (i.e., a vinyl resin) dispersed in phthalate plasticizer;
    2.  liquid plasticizer (i.e., dialkyl phthalate or di-iso-octyl phthalate);
    3. heat and light stabilisers (i.e., liquid barium/cadmium/zinc combined with epoxy plasticizer);
    4.  high proportion of extender to improve wet-on-wet properties.

Printing pastes are made up of four main components:

  1. The coloring matter used (dyes or pigments)
  2. The binding agent
  3. The solvent
  4. The auxiliaries.

The coloring matter used can be either dyestuffs or pigments.   Dyes are in solution and become chemically or physically incorporated into the individual fibers.   The dyes used for printing mostly include vat, reactive, naphthol and disperse colours which have good fastness properties.    Pigments are largely insoluable, so often organic solvents are used (such as benzene or toluene).   The pigmented printing paste must physically bind with the fabric, so must contain a resin, which holds the pigment in place on top of the fabric.

The binder is decisively responsible for the fastness of the pigment prints during use. The most important fastnesses are wash fastness, chemical cleaning fastness and friction fastness. The handle and the brilliance of the colours are also influenced by the choice of binder.
Binders are in general “self-crosslinking polymers” based mainly on acrylates and less commonly on butadiene and vinyl acetate, with solid contents of approx.. 40 – 50%. (2)   Binders made of natural wood resin, wax stand linseed or safflower oils and chitosan were tested in order to obtain biodegradable printing paste.  Promising results were reported when using chitosan as a binder, and no solvent was necessary.

Solvents are usually added in the formulation of the thickeners.  The type of paste (emulsion vs. plastisol) and thickening agent determines the type of solvent needed.  White spirit is a commonly used organic solvent, as is water.  The organic solvent concentration in print pastes may vary from 0% to 60% by weight, with no consistent ratio of organic solvent to water.  Water based solvents may still emit VOC’s from small amounts of solvent and other additives blended into the paste. The liquid waste material of water based pastes may also be considered hazardous waste.

The most important auxiliaries are the thickening agents.  Printing paste normally contains 40 – 70% thickener solution. [3] The printing thickeners used depend on the printing technique and fabric and dyestuff used. Typical thickening agents are starch derivatives, flour, gum Senegal and gum arabic (both very old thickenings, and very expensive today) and albumen. A starch paste is made from wheat starch, cold water, and olive oil, and boiled for thickening.  Starch used to be the most preferred of all the thickenings, but nowadays gums or alginates derived from seaweed is preferred as they allow better penetration of color and are easier to wash out.

Hot water soluble thickening agents as native starch are made into pastes by boiling; the colorants and solvents were added during this step then cooled, after which the various fixing agents would be added.  Colors are reduced in shade by simply adding more stock printing paste.  For example, a dark blue containing 4 oz. of methylene blue per gallon may readily be made into a pale shade by adding to it thirty times its bulk of starch paste or gum, as the case may be. Mechanical agitators are also fitted in these pans to mix the various ingredients together, and to destroy lumps and prevent the formation of lumps, keeping the contents thoroughly stirred up during the whole time they are being boiled and cooled to make a smooth paste. Most thickening agents used today are cold soluble and require less stirring.

Almost exclusively synthetic, acrylate-based thickening agents are used in pigment printing – or none at all, since the mix of resins, solvents and water produces thickening anyway.

Generally, the auxiliaries used for printing are the same as those used in dyeing with a dye bath.  These types of auxiliaries include:

  • Oxidizing  agents (e.g. m-nitrobenzenesulphonate, sodium chlorate, hydrogen peroxide)
  • Reducing  agents (e.g. sodium dithionite, formaldehyde sulphoxylates, thiourea      dioxide, tin(II) chloride)
  • Wetting  agents (nonionic, cationic, anionic)
  • Discharging  agents for discharge printing (e.g. anthraquinone)
  • Humectants   (urea, glycerine, glycols)
  • Carriers:  (cresotinic acid methyl ester,  trichlorobenzene, n-butylphthalimide in combination with other      phthalimides, methylnaphthalene)
  • Retarders  (derivatives of quaternary amines, leveling agents)
  • Resist agents  (zinc oxide, alkalis, amines, complexing agents)
  • Metal  complexes (copper or nickel salts of sarcosine or hydroxyethylsarcosine)
  • Softeners
  • Defoamers,  (e.g. silicon compounds, organic and inorganic esters, aliphatic esters,      etc.)
  • Resins[4]

[1] Ullman’s Fibers, page 766

(2)  Lacasse, K., and Baumann, W., Textile Chemicals: Enviornmental Data and Facts, Springer, 2004; p. 234

[3] Fritz Ullmann, editor,  Ullmann’s Fibers: Textile and dyeing technologies, vol 2; Wiley-VCH Verlag GmbH & Co, KGaA, weinheim, 2008, p. 759

[4] Ulmman, p. 743





Printing – part 1

5 01 2012

It is well known that the “finishing” of a fabric is where a great deal of the environmental impact occurs –  it uses the most water, chemicals and energy.

“Finishing” includes not only the application of treatments to make fabric perform in a certain way (i.e., to be free of something, such as static, wrinkles, or odor, or perhaps be waterproofed or flameproofed).  It also includes the decoration of the fabric.  This decoration can be simply dyeing the fabric a vibrant color.   But glorious designs on fabrics have always been popular.  Applying colored patterns and designs to decorate a finished fabric is called ‘printing’ – and we sure do love them!   Humans have been printing designs onto fabric for centuries.  It has been found on cloth in Egyptian tombs dating to about 5000 B.C. and India exported block prints to the Mediterranean region in the 5th cent. B.C., demonstrating that the Indus Valley civilization knew well the art of dyeing and use of mordents 5,000 years ago.

Printing on fabric is still very much in use today – we could even say it’s wildly popular –  and there’s a lot of talk about the sort of printing inks and dyestuffs used to print fabrics.   So I thought we’d take a look at textile printing and try to find out what the consequences of printing may be to us and the planet.  Printing is one of the most complex of all textile operations, because of the number of variables and the need for a high degree of precision, particularly since there is no way to correct a bad print.  So we’ll be looking at this topic over several weeks.

Technically, printing on textile can be defined as the reproduction of a decoration by application of one tool loaded with coloring material on a textile support. Early forms of textile printing are stencil work, highly developed by Japanese artists, and block printing. In the latter method a block of wood, copper, or other material bearing a design in intaglio (or relief)  with the dye paste applied to the surface is pressed on the fabric and struck with a mallet. A separate block is used for each color, and pitch pins at the corners guide the placing of the blocks to assure accurate repeating of the pattern.

Another style of fabric printing documented in Nuremberg, Germany, was the application of gold or silver dust on still wet fabric. This was an inexpensive way for lesser monasteries and churches to copy the expensive brocades from the Near and Far East, which arrived in Europe via the silk routes. These silk routes most often started in Italy, Venice in particular, and travelled over both land and sea. To economize further in the copying process, color was often filled in areas with a brush, reducing the number of blocks needed.  Velvet  effects were also added sometimes, this was accomplished by spreading powered wool on the gummed ink pattern. The document found in Nuremberg gave specific directions for duplicating the flowers and animals from the brocades.  These procedures could only be used on tapestries, church vestments and table furnishings because the colors weren’t fast. Because they couldn’t be washed these ornate fabrics were not used for clothing.

There are 5 basic steps in printing a fabric:

  1. Preparation of the print paste.
  2. Printing the fabric.
  3. Drying the printed fabric.
  4. Fixation of the printed dye or pigment.
  5. Afterwashing.

We’ll begin with taking a look at  step #2, printing the fabric:  today, a decorative pattern or design is usually applied to constructed fabric by roller, flat screen, or rotary screen methods.

Cylinder or roller printing was developed around 1785.  In the roller printing process, print paste is applied to an engraved roller, and the fabric is guided between it and a central cylinder. The pressure of the roller and central cylinder forces the print paste into the fabric. Because of the high quality it can achieve, roller printing is the most appealing method for printing designer and fashion apparel fabrics.

Screen printing is by far the most popular technology in use today. Screen printing consists of three elements: the screen which is the image carrier; the squeegee; and ink. The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of porous fabric or stainless steel. A stencil is produced on the screen either manually or photochemically. The stencil defines the image to be printed in other printing technologies this would be referred to as the image plate.

In flat bed screen printing, this process is an automated version of the older hand operated silk screen printing. For each color in the print design, a separate screen must be constructed or engraved.

From BBC, Bitesize, Design & Technology, Printing

If the design has four colors, then four separate screens must be engraved. The modern flat-bed screen-printing machine consists of an in-feed device, a glue trough, a rotating continuous flat rubber blanket, flat-bed print table harnesses to lift and lower the flat screens, and a double-blade squeegee trough. The in-feed device allows for precise straight feeding of the textile fabric onto the rubber blanket. As the cloth is fed to the machine, it is lightly glued to the blanket to prevent any shifting of fabric or distortion during the printing process. The blanket carries the fabric under the screens, which are in the raised position. Once under the screens, the fabric stops, the screens are lowered, and an automatic squeegee trough moves across each screen, pushing print paste through the design or open areas of the screens. Remember, there is one screen for each color in the pattern. The screens are raised, the blanket precisely moves the fabric to the next color, and the process is repeated. Once each color has been applied, the fabric is removed from the blanket and then processed through the required fixation process. The rubber blanket is continuously washed, dried, and rotated back to the fabric in-feed area. The flat-bed screen process is a semi-continuous, start-stop operation. Flat screen machines are used today mostly in printing terry towels.

Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber which, however, is prone to wear and edge nicks and has a tendency to warp and distort. While blades continue to be made from rubbers such as neoprene, most are now made from polyurethane which can produce as many as 25,000 impressions without significant degradation of the image.

From a productivity standpoint, the process is slow with production speeds in the range of 15-25 yards per minute. Additionally, the method has obvious design limits. The design repeat size is limited to the width and length dimensions of the flat screen. Also, no continuous patterns such as linear stripes are possible with this method. However, this method offers a number of advantages. Very wide machines can be constructed to accommodate fabrics such as sheets, blankets, bedspreads, carpets, or upholstery. Also, this technique allows for multiple passes or strokes of the squeegee so that large amounts of print paste can be applied to penetrate pile fabrics such as blankets or towels. Currently, approximately 15-18% of printed fabric production worldwide is done on flat-bed screen machines.

Rotary screen printing is so named because it uses a cylindrical screen that rotates in a fixed position rather than a flat screen that is raised and lowered over the same print location. Rotary presses place the squeegee within the screen. These machines are designed for roll-to-roll  printing on fabric ranging from narrow  to wide-format  textiles.

From BBC Bitesize, Design & Technology, Printing

In rotary printing, the fabric travels at a consistent speed between the screen and a steel or rubber impression roller immediately below the screen. (The impression roller serves the same function as the press bed on a flatbed press.) As the fabric passes through the rotary unit, the screen spins at a rate that identically matches the speed of substrate movement.

The squeegee on a rotary press is in a fixed position with its edge making contact with the inside surface of the screen precisely at the point where the screen, substrate, and impression roller come together . Ink is automatically fed into the center of the screen and collects in a wedge-shaped “well” formed by the leading side of the squeegee and the screen’s interior surface. The motion of the screen causes this bead of ink to roll, which forces ink into stencil openings, essentially flooding the screen without requiring a floodbar. The squeegee then shears the ink as the stencil and substrate come into contact, allowing the ink to transfer cleanly to the material.

By converting the screen-printing process from semi-continuous to continuous, higher production speeds are obtained than in flat bed printing. Typical speeds are from 50-120 yards per minute  for rotary screen printing depending upon design complexity and fabric construction.  Rotary screen machines are more compact than flat screen machines for the same number of colors in the pattern. Therefore, they use less plant floor space.

Also with rotary screens, the size of the design repeat is dependent upon the circumference of the screens. This was initially seen as a disadvantage, because the first rotary screens were small in diameter. However, with today’s equipment, screens are available in a range of sizes and are no longer considered design limited. Today’s rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns.

Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide. The principle disadvantage of rotary screen printing is the high fixed cost of the equipment. The machines are generally not profitable for short yardages of widely varying patterns, because of the clean-up and machine down time when changing patterns. Flat screen printing is much more suitable for high pile fabrics, because only one squeegee pass is available with rotary screen. However, rotary machines are used for carpet and other types of pile fabrics.  Most knit fabric is printed by the rotary screen method, because it does not stress (pull or stretch) the fabric during the process.

The rotary garment screen printing machine, developed in the 1960s,  is the most popular device for screen printing in the industry. Screen printing on garments currently accounts for over half of the screen printing activity in the United States. [i]